1. Field of the Invention
The present invention relates generally to improved method and apparatus for deep-sea boundary layer flow measurement, particularly those in which battery operated, autonomous instrumentation is used to make vector velocity measurements and vector velocity component products for long periods of time. In particular, this invention relates to acoustic travel time measurements of three dimensional vector velocity and techniques for extending the deployments of instruments for making such measurements.
2. Description of the Prior Art
Acoustic flow meters for measurement in restricted channels and pipes have developed rapidly in the last twenty years. Measurements of transit time for acoustic pulses over a fixed path have been replaced by measurements of differential transit time for pulses transmitted in opposite directions along a single path. Sequentially measured travel times have been replaced by measurements made with oppositely directed pulses along a single path transmitted simultaneously. Each enhancement has increased the accuracy.
Meanwhile, ocean or open channel flow has been measured by current meters utilizing mechanical, electromagnetic, or acoustic sensors, both Doppler and transit time. Turbulence sensors have been developed for laboratory flow measurements utilizing heated sensors and Doppler scattering of light but these have not been notably successful in autonomous deployments in the ocean due to fouling in the former case and lack of scatterers in the latter case.
An extremely useful configuration of an acoustic travel time sensor for determining three dimensional current vectors of this type is described by the inventor hereof in the paper "BASS, an Acoustic Current Meter Array for Benthic Flow-Field Measurements" by Albert J. Williams 3rd, Marine Geology, vol. 66, pp 345-355, 1985. In that configuration a bottom mounted tripod containing a tower of six acoustic current meters measuring velocity vectors is described. Instantaneous samples were taken every few minutes to obtain a long time series of velocity in a vertical profile.
An alternate arrangement was disclosed in which a rapid burst of data was taken for a short time, limited by data capacity and battery energy. Reynolds stress and turbulent kinetic energy were calculated from the rapid samples. A limitation of this configuration, however, is that no long period measurements of Reynolds stress and turbulent kinetic energy or of true vector velocity means could be measured because of limitations in battery power and recording medium.